BPF Syscall BPF_MAP_UPDATE_ELEM command
The BPF_MAP_UPDATE_ELEM command is used to insert or update a single value in a map.
Return value
This command will return 0 on success or a error number (negative integer) if something went wrong.
Attributes
map_fd
This attribute specifies the file descriptor of the map in which you wish to update.
key
This attribute holds a pointer to the key you wish to update. The size of the key is derived from the key_size attribute of the map you specified with map_fd.
The memory indicated by this field will not be modified.
value
This attribute holds a pointer to a memory location of the value you wish to write to the map. This value should be value_size bytes large for regular maps and be an array of #-logical-CPUs elements of value_size bytes for per-CPU maps.
flags
This attribute is a bitmask of flags.
BPF_ANY
This flag has a value of 0, so setting it together with another flag has no impact. It is meant to be used if no other flags are specified to explicitly state that the command should update the map regardless of if the key already exists or not.
BPF_NOEXIST
If this flag is set, the command will make sure that the given key doesn't exist yet. If the same key already exists when this command is executed the -EEXIST error number will be returned.
Note
Array map types are always pre-allocated and have all keys from 0 to max_entries-1 set to zero, so commands with this flag set will alway fail.
BPF_EXISTS
If this flag is set, the command will make sure that the given key already exists. If no entry for this key exists, the -ENOENT error number will be returned.
BPF_F_LOCK
If this flag is set, the command will acquire the spin-lock of the map value we are updating. If the map contains no spin-lock in its value, -EINVAL will be returned by the command.